The objective is to determine the three-dimensional structure of factor D of the human complement system. Factor D plays an essential role in the initiation and amplification sequence of the alternative complement pathway. Normally, well-defined regulatory mechanisms maintain homeostasis in each step of the 'complement cascade'. However, in a host of pathophysiological conditions, inappropriate activation of the complement cascade occurs, leading to tissue destruction. The goal of this proposal is to obtain precise structural data in atomic detail, and to use this information in the design of specific inhibitors of factor D which help in modulating the complement system. This will be accomplished by the, use of X-ray diffraction methods to determine the crystal structure of factor D and various D/inhibitor complexes. Crystals that are suitable for high-resolution crystallographic analysis have been grown. The structure of D has been determined at 3.2 Angstroms resolution by conventional multiple-isomorphous-replacement and molecular replacement methods and now being refined to high resolution (2.4 Angstroms). The active site and its surroundings will be analyzed by examining the native and inhibitor complex structures using difference-Fourier and computer modelling techniques. This should help in understanding the active site geometry and conformational changes induced in it by inhibitor binding. Good quality crystals of factor D at different pHs (5.8 and 6.8) and an inhibitor complex have been grown and crystallographic studies are underway. Computer modelling organic synthesis and X-ray crystallographic techniques will be used to design effective inhibitors of factor D. It is expected that specific and potent inhibitors of factor D would greatly benefit the cardiovascular patients during open heart surgery and also Kelp in control of chronic tissue destruction as observed in rheumatoid arthritis, systemic lupus, and autoimmune hemolytic anemia.